1. Field of the Invention
The present invention relates to electric power generation control devices for motor vehicles, and more particularly, to techniques for controlling the power generation voltage of an electric power generator in a vehicle equipped with headlights.
2. Description of the Related Art
Generally, a motor vehicle is equipped with an electric generator (alternator) as a source of electric power to be supplied to electrical devices such as headlights. The vehicle is also provided with a battery (electricity storage means) so that electric power may be supplied from the battery to the electrical devices. Further, the electric generator has the function of charging the battery of which the stored electricity is consumed as the electric power is supplied to the electrical devices. The power generation voltage of the electric generator is controlled so that the charged capacity of the battery may become greater than or equal to a target value (target charged capacity). In this type of vehicle, energy regeneration is often carried out. Specifically, during the deceleration, kinetic energy is converted to electric energy to be stored in the battery, and the energy stored in the battery is used during the time other than the deceleration, thereby saving energy. To improve the regeneration efficiency, preferably, the target charged capacity is set low so that the charged capacity of the battery may be as low as possible, and during the deceleration, the power generation voltage is set high.
Japanese Patent No. 3931481 discloses a control device wherein, when an electrical device that consumes much electric power, such as the headlights or the motor of an electric power steering system, is used, the power generation voltage of the electric generator is varied in accordance with the required load of the electrical device.
Meanwhile, the life of the headlight bulb of a vehicle is closely related with the supply voltage applied to the headlight bulb and is inversely proportional, for example, to the 12th to 13th power of the ratio of the supply voltage to the rated voltage. Also, if the supply voltage of the headlight bulb fluctuates, the illuminance of the headlight varies and thus the headlight flickers, making the driver and the people around the vehicle feel uncomfortable. Accordingly, even if the power generation voltage is varied as disclosed in the above patent, the variable range has to be restricted in order to prevent reduction in life of the headlight bulb as well as flickering of the headlight. Especially in far-north regions, it is often the case that the headlights must be turned on even in the daytime, and the variable range of the power generation voltage has to be restricted at all times in such regions. Consequently, only a limited regenerative effect is available, resulting in lowering in fuel efficiency.